The present invention relates to a new process for the preparation of articles formed of polyester resins endowed with valuable mechanical and impact strength properties.
Polyester resins because of their favourable mechanical properties have wide application in the preparation of fibres and films. The resins utilized for this application have values of intrinsic viscosity generally between 0.6 and 0.8 dl/g.
Polyester resins are also used in blow-molding or extrusion-molding processes for the production of bottles and similar thin walled containers.
The resins used in blow molding processes typically have a sufficiently high intrinsic viscosity to allow the realization of the injection operation.
The viscosity suitable for these applications is generally between 0.7-0.8 dl/g and is obtained by solid state upgrading treatments.
For the molding of articles with thick walls or with large dimensions, it is necessary to use resins with a not very high molecular weight to maintain the viscosity in the melt at values suitable for the molding process.
The mechanical properties of the articles thus obtained are not satisfactory, because of the low molecular weight of the resin.
Previously has been tried to obviate this difficulty by carrying out the molding process in the presence of small quantities of polyfunctional compounds blended in the molten resin, which are capable of reacting with the resin end groups to increase the molecular weight of the same.
The use of these polyfunctional compounds results in molded articles in which the polymer has sufficiently high values of intrinsic viscosity to ensure good mechanical properties and impact strength.
Carrying out this process, however, is a difficult operation because of the necessity of using a melt with a relatively high viscosity.
Moreover, it is difficult to control the process and to obtain formed articles with reproducible properties because of the complex reactions which occur in the melt.
It is therefore desirable to have molding processes of easy realization which result in formed articles having high mechanical properties.
A process has now been unexpectedly found for a melt-shaping polyester resins, which results in formed articles endowed of valuable mechanical properties by using in the shaping phase a melt having a not too high viscosity so as to prevent an easy resin processability.
The process of the present invention comprises a first phase in which a melt polyester resin mixed with an upgrading polyfunctional compound capable of increasing the molecular weight of the resin by addition reactions in the solid state with the resin end groups, is melt-shaped under temperature and duration conditions and at concentrations of the upgrading compound such as to avoid increasing the intrinsic viscosity to values greater than about 1.0 dl/g, and a second phase in which the formed article is subjected to an upgrading reaction in the solid state so as to increase the intrinsic viscosity at least 0.1 dl/g with respect to the polymer viscosity before the upgrading treatment.
Preferably, the resin used in the melt-shaping step has an intrinsic viscosity higher than 0.4 dl/g but generally lower than 1.0 dl/g.
The polymer intrinsic viscosity in the manufactured article has values higher than about 0.7 dl/g and generally between 0.7 and 1.8 dl/g.
It has been unexpectedly found that the polymer in the formed articles, having the above intrinsic viscosity, shows particularly high mechanical properties such as tensile strength and resiliency, higher than those of a polymer upgraded with a process different from that of the invention.
Particularly high mechanical properties especially high notched impact strength are shown by the glass reinforced articles.
The invention process is particularly suitable for the preparation of formed articles such as films, panels, plates sheets and generally thick-walled articles.
For the upgrading reaction in the solid state, a thermal treatment is carried out on the formed article, at temperatures higher than about 130xc2x0 C. and lower than the melting point of the resin.
The treatment duration, the temperature and the concentration of the upgrading additive are selected so as to obtain an increase of intrinsic viscosity of at least 0.1 dl/g in comparison with the intrinsic viscosity of the starting polymer used in the melt-shaping step. The upgrading reaction is carried out in an inert gas stream, such as nitrogen, carbon dioxide or, if necessary, under vacuum.
Regarding the films, the mechanical properties are considerably improved if the upgrading treatment is carried under stretch. The utilizable stretching ratios are generally between 1:2 and 1:8.
The polyester resins used in the process of the present invention include products of polycondensation of aromatic bicarboxylic acids such as terephthalic acid or its esters such as dimethylterephthalate, naphthalenbicarboxylic acids, 5-ter butyl-1,3-benzendicarboxylic acids with glycols with 2-10 carbon atoms such as ethylene glycol, 1.4-cyclohexanediol, 1.4 butanediol, hydroquinone.
Polycondensation products are also comprised containing, besides the units deriving from terephthalic acid or its esters, up to 25% of all acid units, units deriving from bicarboxylic acids such as isophthalic and orthophtalic acid. Polyethyleneterephthalate and polybutyleneterephthalate are the preferred resins.
The polyester resin can be added with other compatible polymers such as polycarbonates, elastomeric polyesters and polycaprolactone in amounts up to 20% by weight. Recycled polyester can also be used.
It has been found, and it is another aspect of the invention, that the addition of up to 5% by weight of compounds or polymers having properties of liquid crystals and containing groups reactive with the upgrading compounds, such as the OH and NH2 groups improves the mechanical properties (elastic modulus) of the formed articles (films) subjected to upgrading under stretch. An example of polymer liquid crystals is the product sold by Hoechst Celanese under the trade mark TLCP Vectra A 950.
The upgrading additives usable in the process of the present invention lead to the increase of the resin molecular weight by addition reactions in the solid state, with the end groups of the polyester resin. The additives are used in amount from about 0.05 to 2% by weight on the resin.
Dianhydrides of tetracarboxylic aromatic acids are the preferred additives. The dianhydride of pyromellitic acid is the preferred compound. Other representative dianhydrides are those of 3.3xe2x80x2, 4.4xe2x80x2-benzophenone-tetracarboxy and, 2,2-bis (3,4 dicarboxylphenyl) propane acid, 3.3xe2x80x2-4.4xe2x80x2-biphenyltetracarboxy acid, bis (3.4 dicarboxyphenyl) propane acid 3.3xe2x80x2-4.4xe2x80x2-biphenyltetracarboxy acid, bis (3,4 dicarboxyphenyl) ether, bis (3,4 dicarboxyphenyl) sulfone and their mixtures.
Dianydrides of aliphatic tetracarboxylic acids are also suitable. Examples of these dianydrides are the dianydride of 1,2,3,4 cyclobutanetetracarboxylic acid and 2,3,4,5 tetracarboxyhydrofuran acid.
The blending of the resin with the additive is preferably carried out in a mono or twin screw extruder at a temperature of between 200xc2x0 and 350xc2x0 C.
A counter rotating not intermishing twin screw extruder is preferred.
The extruded polymer is subsequently pelletized and subjected to the shaping process.
The resin is melt-shaped according to the techniques including extrusion blow molding, blow molding, injection-extrusion, extrusion to form sheets, tubes.
Conventional additives can be blended in the polyester. Such additives include stabilizers, antioxidants, plasticizers pigments, nucleant agents, antiflame compounds, inert or reinforcing fillers such as glass fibers.
In general, the addition of the glass fibers improves the notched impact strength, initial flexural modulus and breaking stress of the articles obtained with the process of the present invention. The use of as a little as 5% by weight of glass fibers is sufficient to increase these properties. Concentrations as high as 60% or higher can be used. Preferred concentrations are from 10 and 40% by weight.
The glass fibers are preferably 0.2 to 1 mm in length.
The glass fibers can be added to the composition in any suitable manner. For example they may be incorporated into the polyester resin before the upgrading compound is added or the molten resin, the glass fibers and the upgrading compound are mixed in an extruder. Preferably, the glass fibres are added to the polyester resin premixed with melt with the upgrading compound, the residence time of the resin in the shaping apparatus is lower than 2 minutes, preferably comprised between 20 and 100 seconds, and the barrel temperature is lower than about 300xc2x0 C. and higher than the melting point of the resin. PET is the preferred resin for preparing glass fibers reinforced articles. The glass reinforced resins are particularly suitable for preparing food containers for dual ovenable applications or for any other application involving heat treatment.
The following examples are given to illustrate but not to limit the invention.